The best site I've found for stockings, and hosiery generally, with the largest range of sizes and the best prices, and the best selection of quality suspender belts anywhere (which they manufacture themselves), is:
http://www.nylondreams.co.uk/
The only thing is, you have to ring them up after you place your order to ensure that they actually post your parcel out to you ASAP. Otherwise, your purchases are likely to sit in their offices for days, even up to a week before they're shipped out to you. Aside from that, their customer service is friendly, helpful, and they're very good about returns.
Also, the stockings totally resist laddering like I've never seen! I have rough fingertips from playing music for a living, and they often snag on the stockings, but this seldom results in a ladder on stockings from this company. I've been consistently impressed with how snaggy my fingertips get, and the stockings stay good.
I get most of my stockings and all of my suspender belts from there.